Peggy Hill: the Decline and Fall (2) - S4-E1
Corrected entry: Hank states that DiDi is three years older than Peggy. However, in the episode "Shins of the Father", he said that he went to kindergarten with Didi, which would make him three years older than Peggy as well. This is not possible, though, because he says in "Luanne Virgin 2.0" that he and Peggy were married when they were eighteen. (If he is three years older than Peggy and he was married to her at age eighteen, he would have married a fifteen-year-old).
Correction: She could have gone to school late. Kindergarten isn't mandatory in the US.
Captain Defenestrator
I don't think Didi could have gone to school three years late, though. That would make her twenty-one years old when she graduated from high school, even if she didn't fail a year.
It's quite possible that she only started kindergarten one year late. Then she failed other grades and had to repeat them. The repeated grades would cause her to graduate at age twenty-one. The rules for each school are different, but at my high school, a student could keep attending until age twenty-one.
People have graduated at age 21 or higher in the past. There's no law stating that one can only be in school until the age of 18. Just because YOU doubt it doesn't mean it couldn't have happened.
Captain Defenestrator